The present invention generally relates to a field emission electron microscope, particularly relates to quantitative analysis of the elemental composition of a specimen, and more particularly relates to control of an extracting voltage or an accelerating voltage to be applied to a source.
FIG. 4 shows an example of the conventional configuration of an illumination system of an electron microscope using a field emission source. A field emission current from a field emission source 1 is controlled on the basis of an extracting voltage V.sub.1 applied to a first anode (extracting electrode) 3 in an electrostatic lens 2. A control voltage V.sub.2 for controlling the lens action of the electrostatic lens 2 is applied to a second anode (the first accelerating electrode) 4. A resistor 9 is equally divided so that equal divisional voltages are applied across adjacent electrodes after the second anode 4. A control means for controlling the ratio of the control voltage V.sub.2 for the electrostatic lens 2 to the extracting voltage V.sub.1 to be a fixed value in response to a change of the extracting voltage V.sub.1 is disclosed, for example, in JPA 60-117534. That is, the control voltage V.sub.2 is controlled in response to a change of the extracting voltage V.sub.1. In such a field emission electron microscope, there has been a problem in that when a specimen is illuminated with an electron beam and the characteristic X-rays are detected to thereby perform quantitative analysis of the elemental composition of the specimen, the beam fluctuates making it impossible to perform accurate analysis.